Method for Coating Water-Absorbing Polymer Particles

ABSTRACT

A process for preparing water-absorbing polymer particles by spraying a liquid below the product bed surface by means of at least one spray nozzle in a mixer with moving mixing tools.

The present invention relates to a process for preparing water-absorbingpolymer particles by spraying a liquid below the product bed surface bymeans of at least one spray nozzle in a mixer with moving mixing tools.

The preparation of water-absorbing polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, pages 71 to 103.

Being products which absorb aqueous solutions, water-absorbing polymersare used to produce diapers, tampons, sanitary napkins and other hygienearticles, but also as a water-retaining agent in market gardening.

The properties of the water-absorbing polymers can be adjusted via thedegree of crosslinking. With increasing degree of crosslinking, the gelstrength rises and the centrifuge retention capacity (CRC) falls.

To improve the use properties, for example permeability of the swollengel bed (SFC) in the diaper and absorbency under load (AUL0.9 psi),water-absorbing polymer particles are generally postcrosslinked. Thisincreases only the degree of crosslinking of the particle surface, whichallows absorbency under load (AUL0.9 psi) and centrifuge retentioncapacity (CRC) to be at least partly decoupled. This postcrosslinkingcan be performed in the aqueous gel phase. However, dried, ground andscreened-off polymer particles (base polymer) are preferably coated witha postcrosslinker on the surface, thermally crosslinked and dried.Crosslinkers suitable for this purpose are compounds which comprise atleast two groups which can form covalent bonds with the carboxylategroups of the water-absorbing polymers.

To further improve the permeability (SFC), the particle surface can bemodified further, for example by coating with inorganic inertsubstances, cationic polymers and/or solutions of polyvalent metalcations.

EP 1 191 051 A2 describes spray nozzles with a wide spray angle for usein the postcrosslinking of water-absorbing polymer particles. Thepostcrosslinker solution to be sprayed on preferably has a lowertemperature than the initially charged water-absorbing polymerparticles.

US 2004/0181031 A1 discloses a process for postcrosslinking, wherein thewater-absorbing polymer particles are cooled in an air stream after thethermal postcrosslinking.

It was an object of the present invention to provide an improved coatingprocess for water-absorbing polymer particle.

It was a further object to find a coating process which enables theproduction of very uniformly coated water-absorbing polymer particleswith high permeability (SFC) and low dust content.

It was a further object of the invention to provide a coating processwhich is not prone to disruption.

The object is achieved by a process for producing water-absorbingpolymer particles by polymerizing a monomer solution or suspensioncomprising

-   -   a) at least one ethylenically unsaturated acid-bearing monomer        which may be at least partly neutralized,    -   b) at least one crosslinker,    -   c) optionally one or more ethylenically and/or allylically        unsaturated monomers copolymerizable with the monomers specified        under a) and    -   d) optionally one or more water-soluble polymers,        comprising drying, grinding and classifying, a liquid being        sprayed onto the water-absorbing polymer particles by means of        at least one spray nozzle in a mixer with moving mixing tools,        wherein the liquid is sprayed below the product bed surface.

The product bed surface is the interface which is established betweenthe water-absorbing polymer particles moved within the mixer and theatmosphere above.

The liquid is preferably sprayed at least 10 mm, more preferably atleast 50 mm, most preferably at least 100 mm below the product bedsurface, i.e. the spray nozzle is immersed into the product bed.

Suitable liquids are, as well as pure substances which are liquid at 23°C., also solutions, dispersions, emulsions and melts. The processaccording to the invention is especially suitable for coating withaqueous solutions or dispersions.

The process according to the invention preferably comprises at least onepostcrosslinking. In a particularly preferred embodiment of the presentinvention, the liquid is sprayed onto postcrosslinked polymer particles.

The process according to the invention is not prone to disruption and istherefore particularly suitable for continuous mixers.

The fill level of the mixer is preferably from 30 to 80%, morepreferably from 40 to 75%, most preferably from 50 to 70%.

Too high a water content increases the agglomeration tendency of thewater-absorbing polymer particles. The water content of thewater-absorbing polymer particles to be used in the process according tothe invention is therefore preferably less than 20% by weight, morepreferably less than 10% by weight, most preferably less than 1% byweight.

The temperature of the water-absorbing polymer particles is preferablyfrom 40 to 80° C., more preferably from 45 to 75° C., most preferablyfrom 50 to 70° C.

The liquid is preferably sprayed on by means of a two-substance nozzle,more preferably by means of an internally mixing two-substance nozzle.

Two-substance nozzles enable atomization into fine droplets or a spraymist. The atomization form employed is a circular or else ellipticalsolid or hollow cone. Two-substance nozzles may be configured withexternal mixing or internal mixing. In the externally mixingtwo-substance nozzles, liquid and atomizer gas leave the nozzle headthrough separate orifices. They are mixed in the spray jet only afterthey leave the spray nozzle. This enables independent regulation ofdroplet size distribution and throughput within a wide range. The spraycone of the spray nozzle can be adjusted via the air cap setting. In theinternally mixing two-substance nozzle, liquid and atomizer gas aremixed within the spray nozzle and the biphasic mixture leaves the nozzlehead through the same bore (or through a plurality of bores connected inparallel). In the internally mixing two-substance nozzle, thequantitative ratios and pressure conditions are more highly coupled thanin the externally mixing spray nozzle. Small changes in the throughputtherefore lead to a change in the droplet size distribution. Theadjustment to the desired throughput is effected through the selectedcross section of the nozzle bore.

Useful atomizer gases include compressed air, gas or steam of 0.5 barand more. The droplet size may be adjusted individually via the ratio ofthe liquid to atomizer gas, and also gas and liquid pressure.

In the process according to the invention, it is possible to use allmixers with moving mixing tools known to those skilled in the art, suchas screw mixers, disk mixers, plowshare mixers, paddle mixers, screwbelt mixers, Schugi mixers and continuous flow mixers. The liquid can besprayed on either in high-speed mixers or in mixers with low stirrerspeed.

Mixers with rotating mixing tools are divided into vertical mixers andhorizontal mixers according to the position of the axis of rotation.Advantageously, horizontal mixers are used. A preferred horizontal mixeris the continuous flow mixer.

The residence time in the horizontal mixer is preferably from 1 to 180minutes, more preferably from 2 to 60 minutes, most preferably from 5 to20 minutes.

The peripheral speed of the mixing tools in the horizontal mixer ispreferably from 0.1 to 10 m/s, more preferably from 0.5 to 5 m/s, mostpreferably from 0.75 to 2.5 m/s.

The water-absorbing polymer particles are moved in the horizontal mixerwith a speed which corresponds to a Froude number of preferably from0.01 to 6, more preferably from 0.05 to 3, most preferably from 0.1 to0.7.

For mixers with horizontally mounted mixing tools, the Froude number isdefined as follows:

${Fr} = \frac{\omega^{2}r}{g}$

where

-   -   r: radius of the mixing tool    -   ω: circular frequency    -   g: acceleration due to gravity

In the horizontal mixer, the angle between the mixer axis and the supplyto the spray nozzle is preferably approx. 90°. The liquid can besupplied vertically from the top.

Supply at an oblique angle from the side is likewise possible, in whichcase the angle relative to the vertical is preferably between 60 and90°, more preferably between 70 and 85°, most preferably between 75 and82.5°. The oblique arrangement of the supply enables the use of shortersupply lines and hence lower mechanical stresses during the operation ofthe mixer.

In a particularly preferred embodiment, the spray nozzle is disposedbelow the axis of rotation and sprays in the direction of rotation. As aresult of this arrangement, the coated water-absorbing polymer particlesare conveyed away from the spray nozzle in an optimal manner. Incombination with the oblique arrangement, it is also possible toexchange the spray nozzle during the operation of the mixer without theproduct being discharged.

“Thermally insulated” means that the outer surface of the spray nozzleat least partly has a further material layer, the material of thefurther material layer having a lower thermal conductivity than thematerial of the spray nozzle. The thermal conductivity of the materialof the further material layer at 20° C. is preferably less than 2Wm⁻¹K⁻¹, more preferably less than 0.5 Wm⁻¹K⁻¹, most preferably lessthan 0.1 Wm⁻¹K⁻¹.

“Trace-heated” means that thermal energy is additionally supplied to thespray nozzle, for example by means of electrical energy or by means of aheating jacket flowed through by a heat carrier. Suitable heat carriersare commercial heat carrier oils, such as Marlotherm®, steam or hotwater.

Possible heat supply via one of the feed stocks used in the mixing, i.e.water-absorbing polymer particles or liquid to be sprayed, is not traceheating in the sense of the present invention.

The temperature of the spray nozzle is preferably from 1 to 20° C., morepreferably from 2 to 15° C., most preferably from 5 to 10° C., higherthan the temperature of the water-absorbing polymer particles.

In the case of a thermally insulated spray nozzle, the temperature ofthe liquid to be sprayed is preferably from 1 to 20° C., more preferablyfrom 2 to 15° C., most preferably from 5 to 10° C., higher than thetemperature of the water-absorbing polymer particles. The temperature ofthe liquid to be sprayed corresponds roughly to the temperature of thespray nozzle.

In the case of a trace-heated and optionally thermally insulated spraynozzle, the temperature difference between the water-absorbing polymerparticles and the liquid to be sprayed on is preferably less than 20°C., preferentially less than 10° C., more preferably less than 5° C.,most preferably less than 2° C.

The temperature difference between the liquid to be sprayed on and theatomizer gas is preferably less than 20° C., preferentially less than10° C., more preferably less than 5° C., most preferably less than 2° C.

Suitable liquids are, for example, dispersions of inorganic inertsubstances, solutions or dispersions of cationic polymers, solutions ofdi- or polyvalent metal cations, and also polyols or solutions thereof.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 50 g/100 g of water, and preferably have atleast one acid group each.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids such as acrylic acid, methacrylic acid, maleic acid,fumaric acid and itaconic acid. Particularly preferred monomers areacrylic acid and methacrylic acid. Very particular preference is givento acrylic acid.

The proportion of acrylic acid and/or salts thereof in the total amountof monomers a) is preferably at least 50 mol %, more preferably at least90 mol %, most preferably at least 95 mol %.

The monomers a), especially acrylic acid, comprise preferably up to0.025% by weight of a hydroquinone monoether. Preferred hydroquinonemonoethers are hydroquinone monomethyl ether (MEHQ) and/or tocopherols.

Tocopherol is understood to mean compounds of the following formula

where R¹, is hydrogen or methyl, R² is hydrogen or methyl, R³ ishydrogen or methyl, and R⁴ is hydrogen or an acyl radical having from 1to 20 carbon atoms.

Preferred radicals for R⁴ are acetyl, ascorbyl, succinyl, nicotinyl andother physiologically compatible carboxylic acids. The carboxylic acidsmay be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol where R¹=R²=R³=methyl, inparticular racemic alpha-tocopherol. R¹ is more preferably hydrogen oracetyl. RRR-alpha-tocopherol is especially preferred.

The monomer solution comprises preferably at most 130 ppm by weight,more preferably at most 70 ppm by weight, preferably at least 10 ppm byweight, more preferably at least 30 ppm by weight, in particular around50 ppm by weight, of hydroquinone monoether, based in each case onacrylic acid, acrylic acid salts also being considered as acrylic acid.For example, the monomer solution can be prepared by using acrylic acidhaving an appropriate content of hydroquinone monoether.

Crosslinkers b) are compounds having at least two polymerizable groupswhich can be polymerized by a free-radical mechanism into the polymernetwork. Suitable crosslinkers b) are, for example, ethylene glycoldimethacrylate, diethylene glycol diacrylate, allyl methacrylate,trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane, asdescribed in EP 530 438 A1, di- and triacrylates, as described in EP 547847 A1, EP 559 476 A1, EP 632 068 A1, WO 93/21237 A1, WO 2003/104299 A1,WO 2003/104300 A1, WO 2003/104301 A1 and in DE 103 31 450 A1, mixedacrylates which, as well as acrylate groups, comprise furtherethylenically unsaturated groups, as described in DE 103 31 456 A1 andDE 103 55 401 A1, or crosslinker mixtures, as described, for example, inDE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/32962 A2.

Suitable crosslinkers b) are in particular N,N′-methylenebisacrylamideand N,N′-methylenebismethacrylamide, esters of unsaturated mono- orpolycarboxylic acids of polyols, such as diacrylate or triacrylate, forexample butanediol diacrylate, butanediol dimethacrylate, ethyleneglycol diacrylate or ethylene glycol dimethacrylate, and alsotrimethylolpropane triacrylate and allyl compounds such as allyl(meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters,tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allylesters of phosphoric acid and vinylphosphonic acid derivatives, asdescribed, for example, in EP 343 427 A2. Further suitable crosslinkersb) are pentaerythritol diallyl ether, pentaerythritol triallyl ether andpentaerythritol tetraallyl ether, polyethylene glycol diallyl ether,ethylene glycol diallyl ether, glycerol diallyl ether and glyceroltriallyl ether, polyallyl ethers based on sorbitol, and ethoxylatedvariants thereof. In the process according to the invention, it ispossible to use di(meth)acrylates of polyethylene glycols, thepolyethylene glycol used having a molecular weight between 100 and 1000.

However, particularly advantageous crosslinkers b) are di- andtriacrylates of 3- to 20-tuply ethoxylated glycerol, of 3- to 20-tuplyethoxylated trimethylolpropane, of 3- to 20-tuply ethoxylatedtrimethylolethane, in particular di- and triacrylates of 2- to 6-tuplyethoxylated glycerol or of 2- to 6-tuply ethoxylated trimethylolpropane,of 3-tuply propoxylated glycerol or of 3-tuply propoxylatedtrimethylolpropane, and also of 3-tuply mixed ethoxylated orpropoxylated glycerol or of 3-tuply mixed ethoxylated or propoxylatedtrimethylolpropane, of 15-tuply ethoxylated glycerol or of 15-tuplyethoxylated trimethylolpropane, and also of at least 40-tuplyethoxylated glycerol, of at least 40-tuply ethoxylated trimethylolethaneor of at least 40-tuply ethoxylated trimethylolpropane.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample, in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol.

The amount of crosslinker b) is preferably from 0.01 to 15% by weight,more preferably from 0.5 to 10% by weight, most preferably from 1 to 5%by weight, based in each case on the monomer solution.

Examples of ethylenically unsaturated monomers c) which arecopolymerizable with the ethylenically unsaturated, acid-bearingmonomers a) are acrylamide, methacrylamide, crotonamide,dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate,dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate,diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate anddimethylaminoneopentyl methacrylate.

Useful water-soluble polymers d) include polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, polyglycols orpolyacrylic acids, preferably polyvinyl alcohol and starch.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. Therefore, the monomer solution can be freed ofdissolved oxygen before the polymerization by inertization, i.e. flowingthrough with an inert gas, preferably nitrogen. The oxygen content ofthe monomer solution is preferably lowered before the polymerization toless than 1 ppm by weight, more preferably to less than 0.5 ppm byweight.

The preparation of a suitable polymer and also further suitablehydrophilic ethylenically unsaturated monomers a) are described in DE199 41 423 A1, EP 686 350 A1, WO 2001/45758 A1 and WO 2003/104300 A1.

Suitable reactors are kneading reactors or belt reactors. In thekneader, the polymer gel formed in the polymerization of an aqueousmonomer solution is comminuted continuously by, for example,contrarotatory stirrer shafts, as described in WO 2001/38402 A1. Thepolymerization on the belt is described, for example, in DE 38 25 366 A1and U.S. Pat. No. 6,241,928. Polymerization in a belt reactor forms apolymer gel which has to be comminuted in a further process step, forexample in a meat grinder, extruder or kneader.

Advantageously, the hydrogel, after leaving the polymerization reactor,is then stored, for example in insulated vessels, at elevatedtemperature, preferably at least 50° C., more preferably at least 70°C., most preferably at least 80° C., and preferably less than 100° C.The storage, typically for from 2 to 12 hours, further increases themonomer conversion.

In the case of relatively high monomer conversions in the polymerizationreactor, the storage can also be shortened significantly or a storagecan be dispensed with.

The acid groups of the resulting hydrogels have typically been partiallyneutralized, preferably to an extent of from 25 to 95 mol %, morepreferably to an extent of from 50 to 80 mol % and even more preferablyto an extent of from 60 to 75 mol %, for which the customaryneutralizing agents can be used, preferably alkali metal hydroxides,alkali metal oxides, alkali metal carbonates or alkali metalhydrogencarbonates and also mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonium salts. Particularly preferredalkali metals are sodium and potassium, but very particular preferenceis given to sodium hydroxide, sodium carbonate or sodiumhydrogencarbonate and also mixtures thereof.

Neutralization is preferably carried out at the monomer stage. It isdone typically by mixing in the neutralizing agent as an aqueoussolution, as a melt, or else preferably as a solid material. Forexample, sodium hydroxide having a water content of distinctly below 50%by weight can be present as a waxy mass having a melting point of above23° C. In this case, metering as piece material or melt at elevatedtemperature is possible.

However, it is also possible to carry out neutralization after thepolymerization, at the hydrogel stage. It is also possible to neutralizeup to 40 mol %, preferably from 10 to 30 mol % and more preferably from15 to 25 mol % of the acid groups before the polymerization by adding aportion of the neutralizing agent actually to the monomer solution andsetting the desired final degree of neutralization only after thepolymerization, at the hydrogel stage. When the hydrogel is neutralizedat least partly after the polymerization, the hydrogel is preferablycomminuted mechanically, for example by means of a meat grinder, inwhich case the neutralizing agent can be sprayed, sprinkled or poured onand then carefully mixed in. To this end, the gel mass obtained can berepeatedly ground in a meat grinder for homogenization.

The hydrogel is then preferably dried with a belt dryer until theresidual moisture content is preferably below 15% by weight andespecially below 10% by weight, the water content being determined byEDANA (European Disposables and Nonwovens Association) recommended testmethod No. WSP 230.2-05 “Moisture content”. If desired, however, dryingcan also be carried out using a fluidized bed dryer or a heatedplowshare mixer. To obtain particularly white products, it isadvantageous to dry this gel while ensuring rapid removal of theevaporating water. To this end, the dryer temperature must be optimized,the air feed and removal has to be controlled, and sufficient ventingmust be ensured in each case. The higher the solids content of the gel,the simpler the drying, by its nature, and the whiter the product. Thesolids content of the gel before the drying is therefore preferablybetween 30% and 80% by weight. It is particularly advantageous to ventthe dryer with nitrogen or another nonoxidizing inert gas. If desired,however, it is also possible simply just to lower the partial pressureof the oxygen during the drying in order to prevent oxidative yellowingprocesses.

Thereafter, the dried hydrogel is ground and classified, and theapparatus used for grinding may typically be single- or multistage rollmills, preferably two- or three-stage roll mills, pin mills, hammermills or vibratory mills.

The mean particle size of the polymer particles removed as the productfraction is preferably at least 200 μm, more preferably from 250 to 600μm, very particularly from 300 to 500 μm. The mean particle size of theproduct fraction may be determined by means of the EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP220.2-05 “Particle size distribution”, where the proportions by mass ofthe screen fractions are plotted in cumulated form and the mean particlesize is determined graphically. The mean particle size here is the valueof the mesh size which gives rise to a cumulative 50% by weight.

To further improve the properties, the polymer particles may bepostcrosslinked. Suitable postcrosslinkers are compounds which compriseat least two groups which can form covalent bonds with the carboxylategroups of the hydrogel. Suitable compounds are, for example, alkoxysilylcompounds, polyaziridines, polyamines, polyamidoamines, di- orpolyepoxides, as described in EP 83 022 A2, EP 543 303 A1 and EP 937 736A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE35 23 617 A1 and EP 450 922 A2, or β-hydroxyalkylamides, as described inDE 102 04 938 A1 and U.S. Pat. No. 6,239,230.

Additionally described as suitable postcrosslinkers are cycliccarbonates in DE 40 20 780 C1, 2-oxazolidone and its derivatives, suchas 2-hydroxyethyl-2-oxazolidone, in DE 198 07 502 A1, bis- andpoly-2-oxazolidinones in DE 198 07 992 C1, 2-oxotetrahydro-1,3-oxazineand its derivatives in DE 198 54 573 A1, N-acyl-2-oxazolidones in DE 19854 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amide acetals inDE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327 A2 andmorpholine-2,3-dione and its derivatives in WO 2003/31482 A1.

In addition, it is also possible to use postcrosslinkers which compriseadditional polymerizable ethylenically unsaturated groups, as describedin DE 37 13 601 A1.

The amount of postcrosslinker is preferably from 0.01 to 1% by weight,more preferably from 0.05 to 0.5% by weight, most preferably from 0.1 to0.2% by weight, based in each case on the polymer.

In a preferred embodiment of the present invention, polyvalent cationsare applied to the particle surface in addition to the postcrosslinkers.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium and strontium, trivalent cations such as the cationsof aluminum, iron, chromium, rare earths and manganese, tetravalentcations such as the cations of titanium and zirconium. Possiblecounterions are chloride, bromide, sulfate, hydrogensulfate, carbonate,hydrogencarbonate, nitrate, phosphate, hydrogenphosphate,dihydrogenphosphate and carboxylate, such as acetate and lactate.Aluminum sulfate is preferred. Apart from metal salts, it is alsopossible to use polyamines as polyvalent cations.

The amount of polyvalent cation used is, for example, from 0.001 to 0.5%by weight, preferably from 0.005 to 0.2% by weight, more preferably from0.02 to 0.1% by weight, based in each case on the polymer.

The postcrosslinking is typically performed in such a way that asolution of the postcrosslinker is sprayed onto the hydrogel or the drypolymer particles. After the spraying, the polymer particles coated withthe postcrosslinker are dried thermally, and the postcrosslinkingreaction can take place either before or during the drying.

The spraying of a solution of the postcrosslinker is preferablyperformed in mixers with moving mixing tools, such as screw mixers,paddle mixers, disk mixers, plowshare mixers and shovel mixers.Particular preference is given to horizontal mixers such as plowsharemixers and shovel mixers, very particular preference to vertical mixers.Suitable mixers are, for example, Lödige mixers, Bepex mixers, Nautamixers, Processall mixers and Schugi mixers.

The thermal drying is preferably carried out in contact dryers, morepreferably paddle dryers, most preferably disk dryers. Suitable dryersare, for example, Bepex dryers and Nara dryers. Moreover, it is alsopossible to use fluidized bed dryers.

The drying can be effected in the mixer itself, by heating the jacket orblowing in warm air. Equally suitable is a downstream dryer, for examplea staged dryer, a rotary tube oven or a heatable screw. It isparticularly advantageous to mix and dry in a fluidized bed dryer.

Preferred drying temperatures are in the range from 100 to 250° C.,preferably from 120 to 220° C. and more preferably from 130 to 210° C.The preferred residence time at this temperature in the reaction mixeror dryer is preferably at least 10 minutes, more preferably at least 20minutes, most preferably at least 30 minutes.

Subsequently, the postcrosslinked polymer can be classified again.

To further improve the properties, the postcrosslinked polymer particlescan be coated. Suitable coatings for improving the acquisition behaviorand the permeability (SFC) are, for example, inorganic inert substances,organic polymers, cationic polymers and di- or polyvalent metal cations.Suitable coatings for dust binding are, for example, polyols.

Suitable inorganic inert substances are silicates such asmontmorillonite, kaolinite and talc, zeolites, activated carbons,polysilicic acids, magnesium carbonate, calcium carbonate, bariumsulfate, aluminum oxide, titanium dioxide and iron(II) oxide. Preferenceis given to using polysilicic acids, which are divided betweenprecipitated silicas and fumed silicas according to their mode ofpreparation. The two variants are commercially available under the namesSilica FK, Sipernat®, Wessalon® (precipitated silicas) and Aerosil®(fumed silicas) respectively. The inorganic inert substances may be usedas a dispersion in an aqueous or water-miscible dispersant or insubstance.

When the water-absorbing polymer particles are coated with an inorganicinert material, the amount of inorganic inert material used, based onthe water-absorbing polymer particles, is preferably from 0.05 to 5% byweight, more preferably from 0.1 to 1.5% by weight, most preferably from0.3 to 1% by weight.

Suitable organic materials are polyalkyl methacrylates or thermoplasticssuch as polyvinyl chloride.

Suitable cationic polymers are polyalkylenepolyamines, cationicderivatives of polyacrylamides, polyethyleneimines and polyquaternaryamines.

Polyquaternary amines are, for example, condensation products ofhexamethylenediamine, dimethylamine and epichlorohydrin, condensationproducts of dimethylamine and epichlorohydrin, copolymers ofhydroxyethylcellulose and diallyldimethylammonium chloride, copolymersof acrylamide and α-methacryloyloxyethyltrimethylammonium chloride,condensation products of hydroxyethylcellulose, epichlorohydrin andtrimethylamine, homopolymers of diallyldimethylammonium chloride andaddition products of epichlorohydrin to amidoamines. In addition,polyquaternary amines can be obtained by reacting dimethyl sulfate withpolymers such as polyethyleneimines, copolymers of vinylpyrrolidone anddimethylaminoethyl methacrylate or copolymers of ethyl methacrylate anddiethylaminoethyl methacrylate. The polyquaternary amines are availablewithin a wide molecular weight range.

However, it is also possible to generate the cationic polymers on theparticle surface, either through reagents which can form a network withthemselves, such as addition products of epichlorohydrin topolyamidoamines, or through the application of cationic polymers whichcan react with an added crosslinker, such as polyamines or polyimines incombination with polyepoxides, polyfunctional esters, polyfunctionalacids or polyfunctional (meth)acrylates.

It is possible to use all polyfunctional amines having primary orsecondary amino groups, such as polyethyleneimine, polyallylamine andpolylysine. The liquid sprayed by the process according to the inventionpreferably comprises at least one polyamine, for example polyvinylamine.

The cationic polymers may be used as a solution in an aqueous orwater-miscible solvent, as a dispersion in an aqueous or water-miscibledispersant or in substance.

When the water-absorbing polymer particles are coated with a cationicpolymer, the use amount of cationic polymer based on the water-absorbingpolymer particles is preferably from 0.1 to 15% by weight, morepreferably from 0.5 to 10% by weight, most preferably from 1 to 5% byweight.

When a continuous horizontal mixer is used for the coating with acationic polymer, the residence time of the water-absorbing polymerparticles before the cationic polymer is sprayed on is preferably from 2to 50% by weight, more preferably from 5 to 30% by weight, mostpreferably from 10 to 25% by weight, of the total residence time in themixer.

Suitable di- or polyvalent metal cations are Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺,Ti⁴⁺, Mn²⁺, Fe^(2+/3+), Co²⁺, Ni²⁺, Cu^(+/2+), Zn²⁺, Y³⁺, Zr⁴⁺, Ag⁺,La³⁺, Ce⁴⁺, Hf⁴⁺ and Au^(+/3+); preferred metal cations are Mg²⁺, Ca²⁺,Al³⁺, Ti⁴⁺, Zr⁴⁺ and La³⁺; particularly preferred metal cations areAl³⁺, Ti⁴⁺ and Zr⁴⁺. The metal cations may be used either alone or in amixture with one another. Suitable metal salts of the metal cationsmentioned are all of those which have a sufficient solubility in thesolvent to be used. Particularly suitable metal salts have weaklycomplexing anions, such as chloride, nitrate and sulfate. The metalsalts are preferably used as a solution. The solvents used for the metalsalts may be water, alcohols, dimethylformamide, dimethyl sulfoxide andmixtures thereof. Particular preference is given to water andwater/alcohol mixtures, such as water/methanol or water/propyleneglycol.

The liquid sprayed in the process according to the invention preferablycomprises at least one polyvalent metal cation, for example Al³⁺.

When the water-absorbing polymer particles are coated with a polyvalentmetal cation, the amount of polyvalent metal cation used, based on thewater-absorbing polymer particles, is preferably from 0.05 to 5% byweight, more preferably from 0.1 to 1.5% by weight, most preferably from0.3 to 1% by weight.

When a continuous horizontal mixer is used for the coating with acationic polymer and a polyvalent metal cation, the residence time ofthe water-absorbing polymer particles before the polyvalent metal cationis sprayed on is preferably from 1 to 30%, more preferably from 2 to20%, most preferably from 5 to 15%, of the total residence time in themixer. Advantageously, the polyvalent metal cation is metered in beforethe cationic polymer.

Particularly suitable polyols are polyethylene glycols having amolecular weight of from 400 to 20 000 g/mol, polyglycerol, 3- to100-tuply ethoxylated polyols, such as trimethylolpropane, glycerol,sorbitol and neopentyl glycol. Particularly suitable polyols are 7- to20-tuply ethoxylated glycerol or trimethylolpropane, for example PolyolTP 70® (Perstorp AB, Perstorp, Sweden). The latter have the advantage inparticular that they lower the surface tension of an aqueous extract ofthe water-absorbing polymer particles only insignificantly. The polyolsare preferably used as a solution in aqueous or water-miscible solvents.

The liquid sprayed in the process according to the invention preferablycomprises at least one polyol, for example polyethylene glycol.

When the water-absorbing polymer particles are coated with a polyol, theuse amount of polyol, based on the water-absorbing polymer particles, ispreferably from 0.005 to 2% by weight, more preferably from 0.01 to 1%by weight, most preferably from 0.05 to 0.5% by weight.

When a continuous horizontal mixer is used for the coating with acationic polymer and a polyol, the residence time of the water-absorbingpolymer particles before the polyol is sprayed on is preferably from 20to 80%, more preferably from 30 to 70%, most preferably from 40 to 60%,of the total residence time in the mixer. Advantageously, the polyol ismetered in after the cationic polymer.

The abovementioned coatings can, though, also be applied to polymerparticles (base polymer) which have not been postcrosslinked.

The process according to the invention is suitable both for spraying thepostcrosslinker solution and for spraying other abovementioned liquidson to water-absorbing polymer particles.

The process according to the invention enables uniform coating of thewater-absorbing polymer particles and hence the production of polymerparticles with high permeability (SFC) and low dust content.

Moreover, only a low level of agglomerates form in the process accordingto the invention. In addition, the process is not prone to disruptionowing to the reduced blocking tendency of the spray nozzles to be usedin accordance with the invention.

The process according to the invention is also suitable for coating withthermally sensitive substances.

The water-absorbing polymer particles obtainable by the processaccording to the invention have a centrifuge retention capacity (CRC) oftypically at least 15 g/g, preferably at least 20 g/g, preferentially atleast 22 g/g, more preferably at least 24 g/g, most preferably at least26 g/g. The centrifuge retention capacity (CRC) of the water-absorbingpolymer particles is typically less than 60 g/g.

The water-absorbing polymer particles obtainable by the processaccording to the invention have an absorbency under a load of 6.21 kPa(AUL0.9 psi) of typically at least 10 g/g, preferably at least 12 g/g,preferentially at least 14 g/g, more preferably at least 16 g/g, mostpreferably at least 18 g/g, and typically not more than 30 g/g.

The water-absorbing polymer particles obtainable by the processaccording to the invention have a saline flow conductivity (SFC) oftypically at least 100×10⁻⁷ cm³s/g, usually at least 200×10⁻⁷ cm³s/g,preferably at least 300×10⁻⁷ cm³s/g, preferentially at least 350×10⁻⁷cm³s/g, more preferably at least 400×10⁻⁷ cm³s/g, most preferably atleast 450×10⁻⁷ cm³s/g, and typically not more than 700×10⁻⁷ cm³s/g.

The water-absorbing polymer particles are tested by the test methodsdescribed below.

Methods:

The measurements should, unless stated otherwise, be performed at anambient temperature of 23±2° C. and a relative air humidity of 50±10%.The water-absorbing polymer particles are mixed thoroughly before themeasurement.

Particle Size Distribution (PSD)

The particle size distribution of the water-absorbing polymer particlesis determined analogously to the EDANA (European Disposables andNonwovens Association) recommended test method No. WSP 220.2-05“Particle size distribution”.

Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity is determined analogously to the EDANA(European Disposables and Nonwovens Association) recommended test methodNo. WSP 241.2-05 “Centrifuge retention capacity”, the water-absorbingpolymer particles being screened before the measurement to the particlesize range from greater than 300 to 600 μm.

Absorbency Under Load (AUL0.9 Psi)

The absorbency under a load of 6.21 kPa (0.9 psi) of the water-absorbingpolymer particles is determined analogously to the EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP242.2-05 “Absorption under pressure”, using 0.16 g of water-absorbingpolymer particles with a particle size range of from greater than 300 to600 μm instead of 0.9 g of water-absorbing polymer particles, using awire mesh with a mesh width of 149 μm instead of a mesh width of 36 μmas the base plate and using a weight of 63 g/cm² (0.9 psi) instead of aweight of 21 g/cm² (0.3 psi).

Extractables 16 h

The content of extractable constituents in the water-absorbing polymerparticles is determined by the EDANA (European Disposables and NonwovensAssociation) recommended test method No. WSP 270.2-05 “Extractables”.

Saline Flow Conductivity (SFC)

The saline flow conductivity of a swollen gel layer under a pressure of21 g/cm² (0.3 psi) is, as described in EP 0 640 330 A1, determined asthe gel layer permeability of a swollen gel layer of water-absorbingpolymer particles, except that the apparatus described in theaforementioned patent application on page 19 and in FIG. 8 has beenmodified to the effect that the glass frit (40) is not used, the die(39) consists of the same polymer material as the cylinder (37) and nowcomprises 21 bores of equal size distributed uniformly over the entirecontact surface. The procedure and evaluation of the measurement remainunchanged from EP 0 640 330 A1. The flow rate is recorded automatically.

The saline flow conductivity (SFC) is calculated as follows:

SFC[cm³s/g]=(Fg(t=0)×L0)/(d×A×WP),

where Fg(t=0) is the flow rate of NaCl solution in g/s, which isobtained by means of a linear regression analysis of the Fg(t) data ofthe flow determinations by extrapolation to t=0. L0 is the thickness ofthe gel layer in cm, d is the density of the NaCl solution in g/cm³, Ais the area of the gel layer in cm² and WP is the hydrostatic pressureover the gel layer in dyn/cm².

The EDANA test methods are obtainable, for example, from EuropeanDisposables and Nonwovens Association, Avenue Eugène Plasky 157, B-1030Brussels, Belgium.

EXAMPLES

The examples were performed with commercial water-absorbing polymerparticles based on sodium acrylate.

Such water-absorbing polymer particles are commercially available, forexample, from BASF Aktiengesellschaft (trade name HySorb®), fromStockhausen GmbH (trade name Favor®) and from Nippon Shokubai Co., Ltd.(trade name Aqualic®).

The water-absorbing polymer particles used had the following propertyprofile:

CRC: 26.5 g/g AUL0.9 psi: 21 g/g SFC: 120 × 10⁻⁷ cm³s/g Extractables 16h: 7.8% by weight PSD:   >850 μm 0.7% by weight 600-850 μm 31.3% byweight 300-600 μm 50.5% by weight  90-300 μm 17.3% by weight    <90 μm0.2% by weight

Example 1

The water-absorbing polymer particles were coated in a Ruberg DLM350-1500 continuous flow mixer (Gebrüder Ruberg GmbH & Co KG, Nieheim,Germany) by means of an RZD1-H two-substance nozzle (Gebrüder RubergGmbH & Co KG, Nieheim, Germany) with a 50% by weight aqueous solution ofLupamin® 9095 (BASF Aktiengesellschaft, Ludwigshafen, Germany). Lupamin®9095 is a high molecular weight linear polyvinylamine.

The continuous flow mixer had a mixing chamber volume of 140 l. The filllevel of the continuous flow mixer was 60% and the speed was 43 min⁻¹.The Froude number of the moving water-absorbing polymer particles was0.36.

The two-substance nozzle was installed horizontally. The distance fromthe end wall of the continuous flow mixer was 375 mm and the horizontaldistance of the nozzle mouth from the mixer wall was 50 mm. The nozzlemouth was disposed 100 mm below the product bed surface. The spraynozzle was unheated. The initial pressure of the atomizer gas was 4.8bar. The throughput of atomizer gas was 12 kg/h.

The throughput of water-absorbing polymer particles was 180 kg/h. Thetemperature of the water-absorbing polymer particles was 60° C.

The throughput of coating solution was 7.2 kg/h. The temperature of thecoating solution was 20° C.

The continuous flow mixer was operated without disruption for severalhours.

The coated water-absorbing polymer particles were analyzed. The resultsare compiled in Table 1.

Example 2

The procedure of Example 1 was repeated. The initial pressure of theatomizer gas was 2.4 bar. The throughput of atomizer gas was 21 kg/h.

The continuous flow mixer was operated without disruption for severalhours.

The coated water-absorbing polymer particles were analyzed. The resultsare compiled in Table 1.

Example 3

The procedure of Example 1 was repeated. A 0/60 two-substance nozzle(Düsen-Schlick GmbH, Untersiemau, Germany) was used.

The continuous flow mixer was operated without disruption for severalhours.

The coated water-absorbing polymer particles were analyzed. The resultsare compiled in Table 1.

Example 4

The procedure of Example 1 was repeated. A CSL 2.8 two-substance nozzle(Caldyn Apparatebau GmbH, Ettlingen, Germany) was used.

The continuous flow mixer was operated without disruption for severalhours.

The coated water-absorbing polymer particles were analyzed. The resultsare compiled in Table 1.

Example 5 Comparative Example

The procedure of Example 1 was repeated. An SU4 two-substance nozzle(Spraying Systems Deutschland GmbH, Hamburg, Germany) was used. Thetwo-substance nozzle was installed vertically and the nozzle mouth wasdisposed 150 mm above the product bed surface.

After approx. 20 to 30 minutes, product deposits formed at the nozzlemouth. Steady-state operation was not possible. The experiment wasterminated after 30 minutes. The continuous mixer was yet to reach asteady state. The product obtained was inhomogeneous and was notanalyzed.

TABLE 1 Coating with Lupamin ® 9095 Example CRC AUL0.9 psi SFC PSD > 850μm 1 26 g/g 18 g/g 257 × 10⁻⁷ cm³s/g 12% by weight 2 25 g/g 17 g/g 295 ×10⁻⁷ cm³s/g  8% by weight 3 25 g/g 20 g/g 205 × 10⁻⁷ cm³s/g 15% byweight 4 25 g/g 18 g/g 363 × 10⁻⁷ cm³s/g  7% by weight 5*) —**) —**)—**) —**) *)comparative example **)not determined

1. A process for producing water-absorbing polymer particles bypolymerizing a monomer solution or suspension comprising a) at least oneethylenically unsaturated acid-bearing monomer which may be at leastpartly neutralized, b) at least one crosslinker, c) optionally one ormore ethylenically and/or allylically unsaturated monomerscopolymerizable with the monomers specified under a), and d) optionallyone or more water-soluble polymers, comprising drying, grinding, andclassifying, a liquid being sprayed onto the water-absorbing polymerparticles by means of at least one spray nozzle in a mixer with movingmixing tools, wherein the liquid is sprayed below a product bed surface.2. The process according to claim 1, which comprises at least onepostcrosslinking.
 3. The process according to claim 2, wherein theliquid is sprayed onto postcrosslinked polymer particles.
 4. The processaccording to claim 1, wherein the liquid is sprayed at least 10 mm belowthe product bed surface.
 5. The process according to claim 1, wherein acontinuous mixer is used.
 6. The process according to claim 1, whereinthe fill level of the mixer is from 30 to 80%.
 7. The process accordingto claim 1, wherein the water-absorbing polymer particles fed to themixer have a water content of less than 20% by weight.
 8. The processaccording to claim 1, wherein the water-absorbing polymer particles fedto the mixer have a temperature of from 40 to 80° C.
 9. The processaccording to claim 1, wherein the liquid is sprayed on by means of atwo-substance nozzle.
 10. The process according to claim 1, wherein themixer is a horizontal mixer.
 11. The process according to claim 10,wherein the residence time of the water-absorbing polymer particles inthe horizontal mixer is from 1 to 180 minutes.
 12. The process accordingto claim 10, wherein a peripheral speed of the mixing tools in thehorizontal mixer is from 0.1 to 10 m/s.
 13. The process according toclaim 10, wherein the water-absorbing polymer particles are moved in thehorizontal mixer with a speed which corresponds to a Froude number offrom 0.01 to
 6. 14. The process according to claim 1, wherein at leastone spray nozzle is thermally insulated and/or trace-heated.
 15. Theprocess according to claim 14, wherein the temperature of the spraynozzle is from 1 to 20° C. higher than a temperature of awater-absorbing polymer particles.
 16. The process according to claim14, wherein a thermally insulated spray nozzle is used and thetemperature of the liquid to be sprayed is from 1 to 20° C. higher thana temperature of the water-absorbing polymer particles.
 17. The processaccording to claim 14, wherein a trace-heated and optionally thermallyinsulated spray nozzle is used and a temperature difference between thewater-absorbing polymer particles and the liquid to be sprayed on isless than 20° C.
 18. The process according to claim 14, wherein theliquid is sprayed on by means of a two-substance nozzle and atemperature difference between the liquid to be sprayed and an atomizergas is less than 20° C.
 19. The process according to claim 1, whereinthe liquid is an aqueous solution or dispersion.
 20. The processaccording to claim 1, wherein the liquid comprises at least onepolyamine.
 21. The process according to claim 1, wherein the liquidcomprises at least one polyol.
 22. The process according to claim 1,wherein the liquid comprises at least one polyvalent metal cation. 23.The process according to claim 1, wherein the water-absorbing polymerparticles have a centrifuge retention capacity of at least 15 g/g.